1. Field of the Invention
The present invention relates in general to automotive automatic transmissions of a stepwise type that includes planetary gear units and a plurality of frictional elements for obtaining a plurality of stepwisely varied speeds, and more particularly to hydraulic control devices for hydraulically controlling the transmissions.
2. Description of the Related Art
One of the known hydraulic control devices of the above-mentioned type is shown in Japanese Laid-open Patent Application (Tokkai) 2006-275199. In the hydraulic control device of the publication, the hydraulic pressure from a solenoid valve is selectively fed to two frictional elements, which are engaged in different speed ranges, in accordance with a vehicle operating condition. With such arrangement, the number of parts used for assembling the transmission can be reduced. For ease of description, the arrangement by which the number of parts can be reduced will be referred to as “part reduction arrangement” in the following.
In general, in the automotive automatic transmissions, for permitting the motor vehicle to move (for traction) even when the electric power is entirely shut down due to failure of operation of the transmission, a pressure adjusting valve that adjusts the pressure for establishing a given speed upon failure of the transmission is constructed to receive a given hydraulic pressure even when the electric power is entirely shut down.
For ease of description, the system having such a fail-safe function will be referred to as “power shut off fail-safe system” in the following.